1. Technical Field
The invention relates generally to photodetectors, and more particularly, to the use of deep trenches to contact and isolate vertically-stacked photodiodes buried in a semiconductor material.
2. Background Art
Pixel sensors and multiple wavelength pixel sensors are known in the art. Vertically-stacked multiple-wavelength pixel sensors have also been employed to reduce the surface area of the device occupied by such sensors.
For example, referring first to FIG. 1A, a cross-sectional view of a vertically stacked multiple wavelength pixel sensor 10 is shown, such as that disclosed in U.S. Pat. No. 5,965,875 to Merrill. As shown, pixel sensor 10 includes four alternating, oppositely-doped semiconductor layers. The junction between n-type well 20 and p-type well 22 comprises a first photodiode 32. The junction between p-type well 22 and n-type well 24 comprises a second photodiode 34. The junction between n-type well 24 and p-type substrate 26 comprises a third photodiode 36. Each of the first photodiode 32, second photodiode 34, and third photodiode 36 is adapted to respond to a different wavelength of electromagnetic radiation. For example, first photodiode 32 is adapted to respond to blue light of approximately 450 nm, second photodiode 34 is adapted to respond to green light of approximately 550 nm, and third photodiode 36 is adapted to respond to red light of approximately 650 nm. The sensitivity of each photodiode to a particular wavelength is determined, primarily, by its depth within pixel sensor 10, as is known in the art.
A significant drawback of such an arrangement, however, is that the photodiodes 32, 34, 36 are connected in series and of alternating polarity, i.e., first photodiode 32 and third photodiode 36 are of one polarity and second photodiode 34 is of an opposite polarity. Such an arrangement requires modified circuits or voltage ranges and may require PMOS access transistors in addition to the usual NMOS access transistors, which increases and complicates the circuitry of pixel sensor 10.
In order to eliminate these disadvantages of sensor 10 of FIG. 1A, additional wells of alternating, oppositely-charged semiconductor layers may be employed. FIG. 1B shows a pixel sensor 110 having six alternating, oppositely-charged semiconductor layers. As in FIG. 1A, the junction between n-type well 120 and p-type well 122 comprises first photodiode 132. However, unlike sensor 10 of FIG. 1A, second photodiode 134 comprises p-type well 122, n-type well 124, and p-type well 126. P-type wells 122, 126 act as the anode and n-type well 124 acts as the cathode of second photodiode 134. Similarly, third photodiode 136 comprises p-type wells 126, 130 acting as the anode and n-type well 128 acting as the cathode. As in FIG. 1A, p-type well 130 may be a semiconductor substrate or another p-type well.
In order to ensure that each photodiode has the same polarity, the output 142, 144, 146 of each photodiode 132, 134, 136 is taken from the n-type cathode 120, 124, 128, while the p-type anodes 122, 126, 130 are coupled to a fixed potential such as a ground 140. Thus, pixel sensor 110 avoids the drawbacks associated with serially-connected photodiodes of alternating polarity.
However, significant drawbacks remain in devices such as that of FIG. 1B. Crosstalk between adjacent sensors is common, due to their lack of isolation. In addition, the fact that the upper-most layer in known devices is an n-type layer (20 in FIG. 1A; 120 in FIG. 1B) leads to electron generation at the surface of the sensor 10, 110. Surface electron generation increases dark current in a sensor.
Further, sensor 110 still relies on “reachthrough” diffusions. Reachthrough diffusions suffer from at least two significant drawbacks. First, in order to efficiently contact photodiodes buried deep in a semiconductor substrate, the columns of dopant, e.g., the vertical portions of 120, 122, etc. (FIG. 1B), must be heavily doped Second, in order to introduce the dopant deep enough into the substrate, high-energy implants or long, high-temperature anneals must be used. Both high dopant concentrations and high implant energies create damage to the silicon, increasing dark current and thus degrading the photodiode signal-to-noise ratio. Further, high energy implants and long, high temperature furnace anneals will result in wide columns of dopant, with the width of the column being proportional to the depth. Thus a large pixel area penalty must be paid the for the use of reachthrough diffusions as the photodiode contacting method.
To this extent, a need exists for photodiodes and related structures that do not suffer from the defects described above.